1. Field of the Invention
Embodiments of the present invention generally relate to the fabrication of integrated circuits and particularly to the deposition of an amorphous carbon layer on a semiconductor substrate.
2. Description of the Related Art
Integrated circuits have evolved into complex devices that can include millions of transistors, capacitors and resistors on a single chip. The evolution of chip design continually requires faster circuitry and greater circuit density. The demand for faster circuits with greater circuit densities imposes corresponding demands on the materials used to fabricate such integrated circuits. In particular, as the dimensions of integrated circuit components are reduced to sub-micron dimensions, it has been necessary to use not only low resistivity conductive materials such as copper to improve the electrical performance of devices, but also low dielectric constant insulating materials, often referred to as low-k materials. Low-k materials generally have a dielectric constant of less than 4.0.
Producing devices having low-k materials with little or no surface defects or feature deformation is problematic. Low-k dielectric materials are often porous and susceptible to being scratched or damaged during subsequent process steps, thus increasing the likelihood of defects being formed on the substrate surface. Low-k materials are often brittle and may deform under conventional polishing processes, such as chemical mechanical polishing (CMP). One solution to limiting or reducing surface defects and deformation of low-k materials is the deposition of a hardmask over the exposed low-k materials prior to patterning and etching. The hardmask prevents damage and deformation of the delicate low-k materials. In addition, a hardmask layer may act as an etch mask in conjunction with conventional lithographic techniques to prevent the removal of a low-k material during etch.
Typically, the hardmask is an intermediate oxide layer, e.g., silicon dioxide or silicon nitride. However, some device structures already include silicon dioxide and/or silicon nitride layers, for example, damascene structures. Such device structures, therefore, cannot be patterned using a silicon dioxide or silicon nitride hardmask as an etch mask, since there will be little or no etch selectivity between the hardmask and the material thereunder, i.e., removal of the hardmask will result in unacceptable damage to underlying layers. To act as an etch mask for oxide layers, such as silicon dioxide or silicon nitride, a material must have good etch selectivity relative to those oxide layers. Amorphous hydrogenated carbon is a material used as a hardmask for silicon dioxide or silicon nitride materials.
Amorphous hydrogenated carbon, also referred to as amorphous carbon and denoted a-C:H, is essentially a carbon material with no long-range crystalline order which may contain a substantial hydrogen content, for example on the order of about 10 to 45 atomic %. a-C:H is used as hardmask material in semiconductor applications because of its chemical inertness, optical transparency, and good mechanical properties. While a-C:H films can be deposited via various techniques, plasma enhanced chemical vapor deposition (PECVD) is widely used due to cost efficiency and film property tunability. In a typical PECVD process, a hydrocarbon source, such as a gas-phase hydrocarbon or vapors of a liquid-phase hydrocarbon that have been entrained in a carrier gas, is introduced into a PECVD chamber. A plasma-initiated gas, typically helium, is also introduced into the chamber. Plasma is then initiated in the chamber to create excited CH— radicals. The excited CH— radicals are chemically bound to the surface of a substrate positioned in the chamber, forming the desired a-C:H film thereon.
FIGS. 1A-1E illustrate schematic cross-sectional views of a substrate 100 at different stages of an integrated circuit fabrication sequence incorporating an a-C:H layer as a hardmask. A substrate structure 150 denotes the substrate 100 together with other material layers formed on the substrate 100. FIG. 1A illustrates a cross-sectional view of a substrate structure 150 having a material layer 102 that has been conventionally formed thereon. The material layer 102 may be a low-k material and/or an oxide, e.g., SiO2.
FIG. 1B depicts an amorphous carbon layer 104 deposited on the substrate structure 150 of FIG. 1A. The amorphous carbon layer 104 is formed on the substrate structure 150 by conventional means, such as via PECVD. The thickness of amorphous carbon layer 104 is variable depending on the specific stage of processing. Typically, amorphous carbon layer 104 has a thickness in the range of about 500 Å to about 10000 Å. Depending on the etch chemistry of the energy sensitive resist material 108 used in the fabrication sequence, an optional capping layer (not shown) may be formed on amorphous carbon layer 104 prior to the formation of energy sensitive resist material 108. The optional capping layer functions as a mask for the amorphous carbon layer 104 when the pattern is transferred therein and protects amorphous carbon layer 104 from energy sensitive resist material 108.
As depicted in FIG. 1B, energy sensitive resist material 108 is formed on amorphous carbon layer 104. The layer of energy sensitive resist material 108 can be spin-coated on the substrate to a thickness within the range of about 2000 Å to about 6000 Å. Most energy sensitive resist materials are sensitive to ultraviolet (UV) radiation having a wavelength less than about 450 nm, and for some applications having wavelengths of 245 nm or 193 nm.
A pattern is introduced into the layer of energy sensitive resist material 108 by exposing energy sensitive resist material 108 to UV radiation 130 through a patterning device, such as a mask 110, and subsequently developing energy sensitive resist material 108 in an appropriate developer. After energy sensitive resist material 108 has been developed, the desired pattern, consisting of apertures 140, is present in energy sensitive resist material 108, as shown in FIG. 1C.
Thereafter, referring to FIG. 1D, the pattern defined in energy sensitive resist material 108 is transferred through amorphous carbon layer 104 using the energy sensitive resist material 108 as a mask. An appropriate chemical etchant is used that selectively etches amorphous carbon layer 104 over the energy sensitive resist material 108 and the material layer 102, extending apertures 140 to the surface of material layer 102. Appropriate chemical etchants include ozone, oxygen or ammonia plasmas.
Referring to FIG. 1E, the pattern is then transferred through material layer 102 using the amorphous carbon layer 104 as a hardmask. In this process step, an etchant is used that selectively removes material layer 102 over amorphous carbon layer 104, such as a dry etch, i.e. a non-reactive plasma etch. After the material layer 102 is patterned, the amorphous carbon layer 104 can optionally be stripped from the substrate 100. In a specific example of a fabrication sequence, the pattern defined in the a-C:H hardmask is incorporated into the structure of the integrated circuit, such as a damascene structure. Damascene structures are typically used to form metal interconnects on integrated circuits.
Device manufacturers using a-C:H hardmask layers demand two critical requirements to be met: (1) very high selectivity of the hardmask during the dry etching of underlying materials and (2) high optical transparency in the visible spectrum for lithographic registration accuracy. The term “dry etching” generally refers to etching processes wherein a material is not dissolved by immersion in a chemical solution and includes methods such as reactive ion etching, sputter etching, and vapor phase etching. Further, for applications in which a hardmask layer is deposited on a substrate having topographic features, an additional requirement for an a-C:H hardmask is that the hardmask layer conformally covers all surfaces of said topographic features.
Referring back to FIGS. 1A-E, to ensure that amorphous carbon layer 104 adequately protects material layer 102 during dry etching, it is important that amorphous carbon layer 104 possesses a relatively high etch selectivity, or removal rate ratio, with respect to material layer 102. Generally, an etch selectivity during the dry etch process of at least about 10:1 or more is desirable between amorphous carbon layer 104 and material layer 102, i.e., material layer 102 is etched ten times faster than amorphous carbon layer 104. In this way, the hardmask layer formed by amorphous carbon layer 104 protects regions of material layer 102 that are not to be etched or damaged while apertures 140 are formed therein via a dry etch process.
In addition, a hardmask that is highly transparent to optical radiation, i.e., light wavelengths between about 400 nm and about 700 nm, is desirable in some applications, such as the lithographic processing step shown in FIG. 1B. Transparency to a particular wavelength of light allows for more accurate lithographic registration, which in turn allows for very precise alignment of mask 110 with specific locations on substrate 100. The transparency of a material to a given frequency of light is generally quantified as the absorption coefficient of a material, which is also referred to as the extinction coefficient. For example, for an a-C:H layer that is approximately 6000 Å to 7000 Å thick, the a-C:H layer should have an absorption coefficient of 0.12 or less at the frequency of light used for the lithographic registration, for example 630 nm, otherwise mask 110 may not be aligned accurately. Producing a layer with an absorption coefficient of 0.12 or less may be accomplished by modulating deposition parameters, such as substrate temperature or plasma ion energy.
However, there is typically a trade-off between creating an a-C:H film that possesses high transparency and one with high etch selectivity. An amorphous carbon layer with better etch selectivity will generally have worse transparency. For example, when deposition temperature is used as the modulating factor, a-C:H films deposited at relatively high temperatures, i.e. >500° C., typically possess good etch selectivity but low transparency. Lowering the deposition temperature, especially below 400° C., improves the transparency of the a-C:H film but results in a higher etching rate for the film and, hence, less etch selectivity.
As noted above, in some applications, a hardmask layer may be deposited on a substrate with an underlying topography, for example an alignment key used to align the patterning process. In these applications, an a-C:H layer that is highly conformal to the underlying topography is also desirable. FIG. 2 illustrates a schematic cross-sectional view of a substrate 200 with a feature 201 and a non-conformal amorphous carbon layer 202 formed thereon. Because non-conformal amorphous carbon layer 202 does not completely cover the sidewalls 204 of feature 201, subsequent etching processes may result in unwanted erosion of sidewalls 204. The lack of complete coverage of sidewalls 204 by non-conformal amorphous carbon layer 202 may also lead to photoresist poisoning of the material under non-conformal carbon layer 202, which is known to damage electronic devices. Conformality of a layer is typically quantified by a ratio of the average thickness of a layer deposited on the sidewalls of a feature to the average thickness of the same deposited layer on the field, or upper surface, of the substrate.
Further, it is important that the formation of a hardmask layer does not deleteriously affect a semiconductor substrate in other ways. For example, if, during the formation of a hardmask, a large numbers of particles that can contaminate the substrate are generated, or the devices formed on the substrate are excessively heated, the resulting problems can easily outweigh any benefits.
Therefore, there is a need for a method of depositing a material layer useful for integrated circuit fabrication which has good etch selectivity with oxides, has high optical transparency in the visible spectrum, can be conformally deposited on substrates having topographic features, and can be produced at relatively low temperatures without generating large numbers of particles.